1. Field of the Invention
The present invention relates to venturis which induce the flow of a fluid when an inducing flow of another fluid is passed therethrough. The invention further relates to industrial burners, and in particular to burners which utilize venturis to induce the flow of one or more of the components of a combustible mixture and thereby create such mixture for introduction into a combustion zone. The invention also relates to burner devices capable of creating and handling oxygen rich combustible mixtures.
2. The State of the Prior Art
Venturi devices for inducing the flow of one fluid (the induced fluid) by flow of another fluid (the inducing fluid) are known. These devices generally consist of a tube which has an inlet end, a throat area and an outlet end. Generally speaking, the throat has a smaller flow area than the inlet end whereby to provide a low pressure area at the throat. The inducing fluid flows through the tube from the inlet end of the venturi to the outlet end, and a source of the induced fluid is in fluid communication with the low pressure area created in the throat of the device by the flow of the inducing fluid. Thus, the induced fluid is drawn into the throat and mixes with the inducing fluid.
Venturi devices are particularly useful in burners where a flow of fluid fuel is used to induce a flow of air to thereby create a mixture of the fuel and the air in the venturi. Sometimes, however, it is useful to use the combustion air to induce a flow of the fuel. Alternatively, a flow of air or fuel through the venturi may be used to induce a flow of recirculated flue gas or other diluent to control flame temperature and thus influence NOx production.
In spite of their wide spread use, venturis still have certain limitations. In the first place, the capacity of the venturi for inducing a flow of induced fluid is limited by the available pressure of the inducing fluid and the quantity of the latter needed for a given application. In addition, the length of an efficient venturi typically is directly related to the diameter of the throat. The physical dimensions of the work environment thus may have a limiting influence on the capacity of the venturi.
In a more general sense, the reduction and/or abatement of NOx in industrial burners has always been a desirable aim. Some NOx abatement has been achieved in the past by using a fuel lean primary combustible fuel/air mixture coupled with staging of a portion of the gaseous fuel. Fuel lean primary mixtures are potentially desirable in some applications because the excess air provides a load to reduce flame temperatures whereby to reduce NOx. Staged gas may then be introduced into the combustion zone either from gas tips arranged around the periphery of the burner or from a center gas tip which protrudes through the center of the downstream end of the burner nozzle. The secondary fuel is combusted with the excess air in an environment where flue gases are available as a diluent. These arrangements have not always been successful in reducing NOx to desirable levels.
In some instances, a fuel lean primary mixture is introduced into the combustion zone at a relatively high velocity due to the extra mass provided by the excess air. Such velocity may sometimes be so high that the flame speed is exceeded providing an unstable flame environment.
In accordance with the principles and concepts of the invention, the same provides, in one important aspect, a compound venturi structure which includes a venturi cluster made up of a plurality of venturis. Thus, by definition, in accordance with this aspect of the invention, the compound venturi structure has at least two venturis. Desirably, the structure may have at least three, often will have at least six, and in some instances, depending upon the exigencies of a particular application, may have even more than six venturis. An important purpose of the present invention is to provide practical solutions for problems that are extant in the burner field today, in particular those that involve the production of excessive NOx levels. Thus, the invention provides structure and methodology directed to addressing and alleviating the problems which have been mentioned above. Moreover, the invention solves problems that relate to venturis generally. Because of the increased surface area provided by the multiplicity of venturis, a given volume of the inducing fluid may educe a greater flow of the induced material. Moreover, for a given flow of inducing fluid, the throats of the venturis in a bundle have smaller throats and therefore may be smaller in length.
Each of the venturis of the cluster may have an inlet, a throat and an outlet, and each may be arranged and adapted for causing the flow of an induced material by passage of an inducing fluid therethrough. This action creates, in each venturi, a respective mixture of induced material and inducing fluid, which mixture may then be discharged from the outlets of the respective venturis. The structure also may desirably include a collector having an inlet end which is connected to and arranged in fluid communication with the outlets of the venturis. Thus, the respective mixtures of inducing fluid and induced material discharged from the outlets may be collected and intermixed to present a single mixed stream for discharge from an outlet end of the collector. The induced material most often may be a fluid material; however, in accordance with the broader aspects and contemplations of the invention, the induced material may be a solid flowable material, such as, for example, a powder or a flake material.
The venturis of the compound venturi structure of the invention may desirably, but not necessarily, be in the form of elongated, essentially straight tubes. Preferably, but not necessarily, the tubes may be arranged in essential parallelism relative to one another. The venturis may also have essentially the same physical capacity; however, this also is not a necessary or critical feature of the invention, and in fact, there are many applications where it may be desirable for at least one of the venturis of a given cluster to have a different physical capacity than another of the venturis of that same cluster.
In another important aspect of the invention, the compound venturi structure may be a component of a novel burner assembly. In accordance with this aspect of the invention, in addition to the venturi cluster and the collector, the burner assembly may include a burner tip that is attached to and in fluid communication with an outlet end of the collector. Thus, the tip may be arranged for receiving the single mixed stream of fluids from the collector and directing the same into a combustion zone.
In one important embodiment of the invention, the tip may be elongated and adapted and arranged for directing the single mixed stream out of the tip and into the combustion zone in a generally radial direction relative to a longitudinal axis of the tip. Such a tip may desirably be configured so as to create a round flat flame which surrounds the tip.
In another important embodiment of the invention, the tip may be elongated and adapted and arranged for directing the single mixed stream out of tip and into the combustion zone in a generally axial direction relative to a longitudinal axis of the tip. This tip may desirably be configured so as to create a cylindrical flame which extends along the axis.
In a general sense, either a gaseous fuel or air may be the inducing fluid; however, desirably, at least one of the venturis may be adapted and arranged for operation with a gaseous fuel as the inducing fluid. When a gaseous fuel is used as the inducing fluid, either air or recirculated flue gas may be the induced fluid. Desirably, at least one of the venturis may be adapted and arranged to operate with air as the induced fluid. Thus, when a gaseous fuel is used as the inducing fluid and air is the induced fluid, the single mixed stream created in the collector may comprise a mixture of fluid fuel and air. Similarly, when a gaseous fuel is used as the inducing fluid and recirculated flue gas is the induced fluid, the single mixed stream may comprise a mixture of fluid fuel and flue gas. For some applications, a gaseous fuel may be used as the inducing fluid to induce a flow of air in one venturi of a given cluster and to induce a flow of flue gas in another venturi of the cluster. The single mixed stream may thus comprise a mixture of fluid fuel, air and recirculated flue gas. One or more of the venturis of the cluster may be adapted and arranged to operate with a diluent as the induced fluid, whereby the single mixed stream comprises a fluid fuel and a diluent. The diluent may be steam or nitrogen or CO2 or some other available gas which is inert relative to the combustion reaction process.
In accordance with an important aspect of the invention, the collector may preferably be elongated and arranged so as to include a central axis which extends between the ends thereof. Desirably, the assembly may also include a central fuel tube that extends through the collector along the axis of the latter. Ideally, the central fuel tube may also extend through the burner tip and the same may have a downstream end portion which projects through a centrally located opening at a downstream end of the burner tip. In accordance with a preferred aspect of the invention, the assembly may include a fuel nozzle located at the downstream end portion of the central fuel tube.
Ideally, the inlet end of the collector may include a respective open segment for each of the venturis of the cluster, and the outlets of the venturis may each be connected to a respective segment. The segments may be arranged in a series extending around the central fuel tube so that the mixed streams are evenly distributed around the interior of the collector. If the tip is adapted and arranged for directing the single mixed stream out of the tip and into the combustion zone in a generally radial direction relative to a longitudinal axis of the tip, the fuel nozzle may desirably be adapted and arranged for providing secondary fuel to the combustion zone. On the other hand, if the tip is adapted and arranged for directing the single mixed stream out of the tip and into the combustion zone in a generally axial direction relative to a longitudinal axis of the tip, the fuel nozzle may desirably be adapted and arranged to provide a continuous primary flame at a location in the zone which is spaced axially from the downstream end of the tip. Ideally, in the latter case, the fuel nozzle may be located at a position where it is spaced far enough from the downstream end of the tip in the combustion zone such that the single mixed stream has been allowed to expand and slow to a speed such that its velocity, when it comes into proximity with the fuel nozzle, is no greater than the flame sustaining velocity.
In another aspect, the invention provides a burner assembly that comprises a burner tube structure which may, but does not necessarily, include one or more venturi tubes. The burner tube structure does, however, include an elongated burner conduit having spaced inlet and outlet ends. Such conduit may be a venturi tube. Alternatively it may simply be a hollow tube or pipe. The conduit may generally be adapted and arranged for directing a combustible gaseous mixture comprising a fluid fuel, preferably in the form of a gaseous fuel, and oxygen, preferably in the form of air, therealong from the inlet end thereof to the outlet end. In accordance with this aspect of the invention, a burner tip may be provided at the outlet end of the conduit, and such burner tip may desirably have a central axis and a downstream end spaced from the outlet end of the conduit. The tip may generally be arranged and adapted for receiving the combustible mixture from the conduit and directing the same through one or more apertures at the downstream end of the tip and into a combustion zone in a direction generally along the axis of the tip.
The assembly of this aspect of the invention may further include an elongated central fuel tube that extends through the tip and along the axis. This fuel tube desirably may project out of the tip in an axial direction through the downstream end of the latter, and the fuel tube may have a downstream end portion that is located in the combustion zone in spaced relationship relative to the downstream end of the burner tip. The aperture or apertures at the downstream end of the tip may be disposed around the fuel tube, whereby the mixture directed into the combustion zone may generally be in the form of a cylinder which surrounds the fuel tube and extends outwardly of the downstream end of the tip along the axis toward the downstream end portion of the fuel tube. Ideally, the assembly includes a fuel nozzle on the downstream end portion of the fuel tube which is located at a position in the zone that is sufficiently remote from the downstream end of the burner tip so as to permit the mixture to expand after it has left the downstream end of the tip and slow to a velocity which is less than the flame velocity thereof before it comes into proximity with the fuel nozzle. In this form of the invention, the burner assembly may desirably be used in situations where the combustible mixture comprises an ultra fuel lean mixture of fuel and air.
In further accordance with the concepts and principles of the invention, a generally dome shaped burner tip is provided. The novel burner tip of the invention desirably includes a generally ring shaped base portion having a central axis and a plurality of elongated, side-by-side, circumferentially spaced, longitudinally curved ribs which extend in a direction along the axis. The ribs may each have a first end that is mounted on the base and a second end that is spaced from the base, with the second ends being located nearer the axis than the first ends. The base portion and the ribs together define an area inside the tip adapted for receiving a flow of a mixture of air and fluid fuel, and the ribs alone define a multiplicity of curved slots therebetween permitting the mixture to flow from the area inside the tip and outwardly into a combustion zone outside the burner tip in both a radial direction and in a direction which includes a vector extending along the axis. In accordance with the invention, the burner tip may comprise a crown portion connected to the second ends of the ribs, and such crown portion may include a plurality of axially and radially extending discontinuities which are aligned with respective slots such that the air/fluid fuel mixture flowing through the discontinuities has a more pronounced axial flow direction relative to the air/fluid fuel mixture flowing through the slots. These discontinuities may desirably be positioned so as to cause the air/fluid fuel mixture flowing therethrough to create a prestaged mixing area outside the combustion zone. The crown portion may also have an axially aligned, gas nozzle accommodating opening therein.
In one preferred embodiment of the invention, the tip described in the foregoing paragraph may be used in conjunction with a burner assembly that comprises a compound venturi structure as described above.
The invention also provides a method for increasing the capacity of a venturi device to induce the flow of a second fluid into a first fluid when a flow of the first fluid passes through the device. The method comprises separating the first fluid into at least two, desirably at least three, perhaps at least six or more separate flow portions, passing each separate flow portion of the first fluid through a respective venturi to independently induce a flow of the second fluid into each of the flow portions thereby creating respective separate mixtures of the first and second fluids, and admixing the respective separate mixtures to thereby create an admixture of the first and second fluids containing a greater concentration of the second fluid than would be possible by passing the entire amount of the first fluid through a single venturi. In accordance with the invention, the first fluid may desirably be a gaseous fuel and the second fluid may desirably be air.
The invention further provides a method for decreasing the length of a venturi device adapted for inducing the flow of a second fluid into a first fluid when a flow of the first fluid is passed through the device. In this form of the invention, the method comprises separating the first fluid into at least two, preferably at least three, and perhaps at least six or more separate flow portions; passing each separate flow portion of the first fluid through a respective venturi to independently induce a flow of the second fluid into each of the flow portions of the first fluid, thereby creating respective separate mixtures of the first and second fluids; and admixing the respective separate mixtures to thereby create an admixture of the first and second fluids containing a greater concentration of the second fluid than would be possible by passing the entire amount of the first fluid through a single venturi of the same length.
Furthermore, the invention provides a method for operating a venturi device that comprises providing at least two venturis, each venturi having an inlet, a throat and an outlet, and each being operable for inducing the flow of an induced material when an inducing fluid is passed therethrough, whereby to produce a respective mixture of the induced material and the inducing fluid and discharging the mixture from the outlet thereof; passing a first inducing fluid through a first of the venturis to thereby induce the flow of a first induced material and produce a first mixture comprising the first inducing fluid and the first induced material, and discharging the first mixture from the outlet of the first venturi; passing a second inducing fluid through a second of the venturis to thereby induce the flow of a second induced material and produce a second mixture comprising the second inducing fluid and the second induced material, and discharging the second mixture from the outlet of the second venturi; and collecting and intermixing the first and second mixtures to present a single mixed stream of the fluids and materials.
Additionally the invention provides a method for operating a burner equipped with a venturi device for supplying a combustible mixture to a burner nozzle which comprises providing at least two venturis, each venturi having an inlet, a throat and an outlet, and each being operable for inducing the flow of an induced fluid when an inducing fluid is passed therethrough, whereby to produce a respective mixture of the induced and inducing fluids that is discharged from the outlet thereof; passing a first inducing fluid through a first of the venturis to thereby induce the flow of a first induced fluid and produce a first mixture comprising the first inducing fluid and the first induced fluid, and discharging the first mixture from the outlet of the first venturi; passing a second inducing fluid through a second of the venturis to thereby induce the flow of a second induced fluid and produce a second mixture comprising the second inducing fluid and the second induced fluid, and discharging the second mixture from the outlet of the second venturi; and collecting and intermixing the first and second mixtures to present a single combustible mixed stream of the fluids. Ideally, the first and second inducing fluids may each be gaseous fuels and the first and second induced fluids may each be air. Alternatively, the first induced fluid may be air and the second induced fluid may be a recirculated flue gas or other diluent such as steam or nitrogen or CO2 or any other inert gas.